1. Field of the Invention
The present disclosure relates generally to memory devices and, more specifically, to a technique for salvaging memory devices exhibiting errors.
2. Description of the Related Art
Various phenomena can affect data retention in non-volatile memory (NVM). For example, some causes of limited data retention and data errors are intrinsic, while others are extrinsic. Examples of extrinsic causes of limited data retention and data errors include oxide layer variations and defects, ionic contamination, etc. Since such extrinsic causes affect data retention, and data retention involves retaining data over time, data errors arising from extrinsic causes often take considerable time to occur. When a data error affecting a particular bit has occurred, the state of that bit is the opposite its correct value, that is the incorrect value. Thus, the bit can be said to have flipped.
During flash memory operation (e.g., data retention, operating life, etc.), it is almost inevitable that some extrinsic bits will come out of population, from erase state, from program state, or from both. Error Correction Code (ECC) can be used to correct the extrinsic bits once bit flip happens.
ECC has a capacity limit, e.g., a typical ECC algorithm may not be able to correct double-bit or multi-bit errors from the same ECC section. While ECC typically cannot correct such errors, it can detect all double-bit errors and 70% of triple-bit errors. However, the detection of uncorrectable errors leads to the situation that, whenever these errors happen, a memory part being tested will fail testing, as the inability to correct such errors prevents the memory part from reliably storing data in the locations where the errors occurred.
Failure of a memory part as a result of uncorrectable double-bit or multi-bit errors reduces the yield of the manufacturing process and can reduce reliability of the memory parts overall.
The use of the same reference symbols in different drawings indicates similar or identical items.